Bipolar electrode

ABSTRACT

AN IMPROVED DIMENSIONALLY-STABLE BIPOLAR ELECTRODE FOR USE IN ELECTROCHEMICAL PROCESSES, COMPRISING A VALVE METAL LAYER, SUITABLE ANODIC MATERIAL ON THE ANODE SIDE OF THE VALVE METAL, A BARRIER LAYER OF TUGSTEN ON THE CATHOD SIDE OF THE VALVE METAL, THE TUNGSTEN LAYER COATED WITH A LAYER OF IRON OR NICKEL.

US. Cl. 204--290 F 10 Claims ABSTRACT OF THE DISCLOSURE An improveddimensionally-stable bipolar electrode for use in electrochemicalprocesses, comprising a valve metal layer, suitable anodic material onthe anode side of the valve metal, a barrier layer of tungsten on thecathode side of the valve metal, the tungsten layer coated with a layerof iron or nickel.

This invention relates to improved electrodes for use in electrochemicalprocesses. More particularly, this invention relates to improved,corrosion-resistant, dimensionally-stable bipolar electrodesparticularly suitable for the electrolysis of alkali metal chloridesolutions in the production of alkali metal chlorates.

The electrolysis of aqueous solutions of alkali metal chlorides such assodium chloride or potassium chloride to produce the correspondingchlorate has been practiced commercially on a wide scale.

, Graphite electrodes have been employed in the past in various alkalinechloride electrolysis operations; however, there have been certaindisadvantages which have arisen as a result of the use of graphite. Oneof the most serious of the disadvantages is the constant attrition ofthe graphite during the electrolysis operation. Attrition of thegraphite results in the increase of the clearance or spacing between theanode and cathode, which in turn causes an increase in the cell voltagedrop and a resulting decreasing efiiciency in cell operation.

Graphite electrodes have a limited life, generally being on the order ofabout 1.0 inches in thickness on installation, but at the end of 10-12months of continuous use may be reduced to about 0.25 inches, with theattendant loss in power and efficiency. Such losses, including economiclosses, have resulted in the proposed use of metallic electrodes and theuse of bipolar cells.

Generally, in the production of alkali metal chlorate, bipolar metalelectrodes are now preferentially used which, when arranged suitably inan electrolysis cell, in a spaced electrical series, serve to frunctionboth as the anode and cathode in the cell. The electrodes are subjectedto an electrical potential while immersed in the alkali metal chloridesolution and, electrochemically, alkali metal chlorate is produced,either in the cell itself, or outside the cell upon therstanding of thesolution.

The advantages of the use of bipolar cells and bipolar electrodesinclude:

(a) bipolar cells are relatively simpler and more economical to producethan are monopolar cells;

(b) the electrical contact for supplying current to the electrodes inbipolar cells is applied only through the first and last plates whilethe current supply to the anodes of monopolar cells must be supplied byelectrical contact established with each individual anode;

(c) bipolar cells allow the use of minimal distances between theelectrodes which reduces voltage and allows for a reduction of thevolume of electrolyte employed.

Platinum group metal coated electrolytic valve metals have been proposedas substitutes for graphite electrodes. These metallic electrodes haveofiered several potential advantages over the conventional graphiteelectrodes as, for example, lower overvoltage, lower rates of erosion,

"United States Patent "ice and the resulting electrolysis production ofhigher purity products. The economic advantages gained by the use ofsuch electrodes, however, must be sufficiently high to overcome the highcost of these metallic electrodes.

A problem existent with such electrodes based on anodic precious metalsis that the titanium or other valve metal support is attacked byhydrogen on the cathodic side during electrolysis, forming hydrides andcausing disintegration of the electrode.

The present invention provides a bipolar electrode having excellentelectrolytic use characteristics and excellent durability. Theelectrodes of the present invention are composite bipolar electrodeshaving a valve metal layer, suitable anodic material, preferably aplatinum group metal or metal oxide, deposited on one side of the valvemetal, a barrier layer of tungsten on the cathodic side of the valvemetal, the tungsten layer coated with an outer layer of iron or nickel.

The electrode comprises a central or inner layer of valve metal of whichthe oxide is chemically resistant under anodic conditions to theelectrolyte employed. The expression valve metal as employed herein isdefinitive of a metal which can function generally, as a cathode in anelectrolyte cell, but not generally as an anode, due to the formation,under anodic conditions, of the oxide of the metal, which oxide, oncedeveloped, is highly resistant to the passage therethrough of electrons.

The preferred valve metal is titanium, although tantalum or columbiummay also be advantageously employed.

The expression chemically resistant under anodic conditions hereinbeforeemployed, as applied to the valve metal, indicates that the oxide isresistant to the corrosive surrounding electrolyte and is not, to anappreciable extent, subject to erosion, deterioration or to electrolyticattack.

The tungsten, with the nickel or iron outer coating, has a low hydrogendiffusion rate and effectively prevents the migration of cathodicallyproduced hydrogen from reaching the surface of the valve metal.

The cathodic side of the valve metal layer is adhered to a layer oftungsten. The valve metal, preferably titanium, may be adhered to thetungsten layer by any means available in the art, as for example, byutilizing sputtering techniques. The thickness of the layers is notcritical. Generally, the valve metal layer is on the order of from about0.10 inch to about 0.7 or 0.8 inch in thickness, with the layer of thetungsten being on the order of from about 0.01 inch in thickness up toabout 0.1 inch in thickness, if desired.

At least an operable portion of the opposing face of the valve metallayer has bonded thereto a layer of suitable anodic material chemicallyresistant under anodic conditions to the electrolyte used. The termsuitable anodic material as employed herein refers to a material whichis electrically conductive, resistant to oxidation and substantiallyinsoluble in the electrolyte. Platinum is the preferred anodic material;however, it is also possible to utilize ruthenium, palladium, osmium,iridium, oxides of these materials, alloys of two or more of the metals,or suitable mixtures thereof.

The iron or nickel layer may be applied to the tungsten layer by anysuitable means known to the art which is non-destructive to the tungstenlayer, e.g., electroplating, vacuum deposition, metal spraying or thelike. The thickness of the iron or nickel layer may vary, but thematerial is generally utilized as a thin film, about 0.01 inch inthickness, or greater.

The anodic material, preferably platinum, can be applied to the anodicside of the valve metal by using such sources of platinum aschloro-platinic acid or a thermallydecomposable metallo-organic compoundsuch as a platinum resinate. For example, the platinum resinate may bemixed with an organic solvent or diluent, such as terpenes or aromatics,typically oil of turpentine, xylene or toluene, before being applied tothe base layer. The layer is heated to decompose and/or to volatilizethe organic matter and other non-metallic components, leaving on thebase member a layer of adherent electroconductive platinum. In producinga metallic anodic coating by such method, care should be taken to avoidoxide formation, for example, by limiting the temperatures of heating orby effecting heating in an oxygen-free atmosphere such as in a vacuum orunder a nitrogen or argon blanket.

Heating may be elfected in an air atmosphere; however, temperaturesabove about 600-650" C. are not recommended due to the possibility ofoxidizing valve metal.

In the production of an anodic oxide coating, temperatures and times ofheating are more selected that will result in the formation of an oxide,preferably an oxide of platinum group metals, such as ruthenium oxide.The temperature applied may vary dependent upon the particular platinumgroup metal oxide. Typically, the temperature may be in the range of 300C. to 600 C., preferably about 350 C. to about 550 C. for a period offrom about minutes to about 2 hours. The heating step is mostadvantageously conducted in an atmosphere containing elemental oxygensuch as air or other oxygeninert gas mixtures although an atmosphere ofpure oxygen could be used. The platinum group oxide formed is eithercrystalline or amorphous depending upon the temperature of heating, withthe degree of crystallinity increasing as temperatures and duration ofheating are increased. Both crystalline and non-crystalline coatingshave good electro-conductivity. Where the coatings have a low degree ofcrystallinity, improved adhesion and conductivity are noted.

It is not necessary that the anodic material completely cover the entiresurface of the central or inner layer. However, the total anodic side ofthe central or inner layer should be coated with the anodic material tothe extent that the massed portion of the anodic material functioneffectively as an anode. It is preferred that the anodic materialessentially cover the anodic side of the valve metal layer.

The anodic layer, preferably a platinum group metal or metal oxide, canbe deposited to the extent of 0.0001 inch, although lesser or greaterthicknesses may be achieved, depending on the methods of deposition, itonly being necessary that the anodic material be present on the anodicside of the valve metal layer in an amount sufii cient to functioneffectively as the anode.

The anodic material, as hereinbefore stated, can be deposited on thecentral or inner layer of valve metal by any suitable method known tothe art. The deposition can be effected, for example, by using a bathconsisting of 4.5 grams platinic chloride and 22 ml. 37 percenthydrochloric acid dissolved in 2800 ml. water. The temperature isgenerally maintained between about 70 and 85 C., and the currentintensity is such that essentially no hydrogen is evolved at the valvemetal panel. A graphite anode is used in the bath and valve metal ismade cathodic. The panel is agitated or moved during the depositionoperation, and the current is regulated as to preclude hydrogeninvolvement at the valve metal, with the anodic platinum being depositedat a thickness of less than about 0.0001 inch. Minor variations may beeffected in the deposition of the precious metal and varying thicknessesmay be obtained by suitable modifications in the time consumed in theelectrodeposition operation. Also, simultaneous deposition may be madeof more than one component, as for example, by effecting a coatingcontaining, in addition to platinum, another platinum group metalcompound such as ruthenium, such other component being added to theelectrodeposition bath whereby a desired composite coating is obtained.

The electrodes of the present invention, as stated, find particularapplication in the electrolysis production'of alkali metal chlorates. Inproducing chlorates using the electrodes of the present invention, theprocess may be carried out continuously by passing a solution containingalkali metal chloride through the cells at temperatures generally on theorder of up to the boiling point of the electrolyte with the effiuentliquor cooled or concentrated to promote crystallization of the chlorateproduced in the cell. Advantageously, a small amount of chromate may beadded to the liquor fed to the cell to promote chlorate formation, inaccordance with methods known in the art.

A typical bipolar electrolytic unit which can be used with the novelelectrodes of the present invention consists of a housing havingspaced-apart end electrodes with the enclosed space defined by the wallsand end electrodes intermediate at intervals by the bipolar electrodesinto substantially isolated unit cells. For each individual electrolysiscell, there is an individual reaction zone and an individualelectrolysis zone substantially isolated from the reaction andelectrolysis zones of the adjoining cells, the term unit cell referringto one of the chambers or sections into which the apparatus is dividedby the bipolar electrodes. Such cell makeup permits of good circulationof the electrolyte between the zones.

A bipolar electrolytic cell utilizing the bipolar electrodes describedhas essentially minimal or no current leakage and voltages on the orderof 3.8 to 4.0 volts can be employed, with a current density of about 4amp./in.

What is claimed is:

1. A bipolar electrode consisting of a layer of valve metal, at least aportion of the anodic surface of which is conductively covered by amaterial selected from the group consisting of platinum, palladium,ruthenium, osmium, iridium, mixtures thereof and oxides thereof, abarrier layer of tungsten on the cathodic side of the valve metal layerand a layer of a material selected from iron and nickel on the exteriorsurface of the tungsten layer.

2. A bipolar electrode as defined by Claim 1 wherein the valve metal istitanium.

3. A bipolar electrode as defined by Claim 1 wherein the anode sidecovering material is platinum metal.

4. A bipolar electrode as defined by Claim 1 wherein the layer on thetungsten is nickel.

5. A bipolar electrode as defined by Claim 1 wherein the layer on thetungsten is iron.

6. A bipolar electrode as defined by Claim 1 wherein the anode sidecovering material is ruthenium oxide.

7. A bipolar electrode as defined by Claim 2 wherein the anode sidecovering material is platinum metal.

8. A bipolar electrode as defined by Claim 2 wherein the anode sidecovering material is ruthenium oxide.

9. A bipolar electrode as defined by Claim 7 wherein the layer on thetungsten is iron.

10. A bipolar electrode as defined by Claim 8 wherein the layer on thetungsten is iron.

References Cited UNITED STATES PATENTS 2,955,999 10/1960 Tirrell 204-1803,291,714 12/1966 Hall et a1. 204-256 3,307,925 3/1967 Jacobson 29-195 A3,380,908 4/1968 Ono et al. 204-290 F 3,491,014 1/1970 Bianchi et a1204-242 3,562,008 2/1971 Martinsons 117-221 3,671,415 6/1972 King et al.20'4-284 3,711,382 l/l973 Anthony 204-1 R 2,711,389 6/1955 Beach et al.20432 J. C. EDMUNDSON, Primary Examiner U.'S. C1. X.'R. 29-196, 198

